The most pertinent prior art known to the applicant is U.S. Pat. No. 3,131,322 issued Apr. 28, 1964 to Pleiss et al.
It has long been desirable to provide various protection schemes for dynamoelectric machines which sense an impending undesirable condition and provide a signal or take action in response to such impending condition to alter the mode of operation of the machine. Perhaps the most common examples are thermal overload protectors employed in motors. In such devices, the temperature of some part of the motor, typically some part of the stator, is subject to continuous monitoring. When the temperature achieves some predetermined elevated value above that which could cause breakdown of winding insulation or other heat related problems, the circuit to the motor is opened in response to the detection of the elevated temperature.
To be complete reliable, the temperature sensor must be properly placed within the machine. While the sensor can be associated with the stator iron, it is frequently preferred to avoid such a location since heat generated in the windings within the stator iron is readily conducted away through the iron to a housing or to a coolant when back iron cooling is employed. Consequently, stator iron temperatures are typically lower than temperatures elsewhere in the windings. Conventional wisdom thus suggests that the sensor be disposed in some relation with the winding end turns. As is well known, the end turns emerge from the stator iron to be substantially in contact only with each other and the ambient air. As a consequence, they will not run as cool as the remainder of the stator with the result that monitoring of the end turn temperature provides a more accurate indication of impending overheating.
The problem then becomes one of properly disposing a thermal sensor in good heat transfer relation with the winding end turns. One approach may involve the disposition of the sensor within the windings after they have been inserted into the stator iron, but prior to varnish impregnation. The difficulty with this approach is that it will frequently involve some deformation of the conductors forming the windings which are typically varnished wires. The deformation may crack or otherwise weaken the varnish with the results that its insulation qualities are lessened. Consequently, the resulting machine may be more prone to failure due to the weakened insulation.
Another approach is to dispose the sensor within the windings as they are being inserted into the stator. This approach will assure an intimate heat transfer association between the windings and the sensor but is subject to other difficulties. For one, in the event of sensor failure, the same cannot be easily removed and replaced without deforming the windings which in turn may cause a loss of insulation qualities. For another, many sensors are sensitive to the physical forces involved in handling and winding and thus may be partially or wholly rendered inoperative during the process of forming the windings about the sensors.
Still another approach is exemplified by the above identified Pleiss et al patent wherein a sensor receiving pocket is disposed in the windings as they are formed. This approach eliminates a number of the difficulties mentioned previously but unless good care is taken, there is a possibility that a metal thermal conductor employed in such a construction may inadvertantly contact the windings. If there is an insulation failure at such point of contact, a short affecting motor performance or leading to a localized hot spot can result.
The present invention is directed to overcoming one or more of the above problems.